As Johnny Thompson bent over the black waters of the river he thought he heard a stealthy movement behind him. Before he could decide whether or not his eyes had deceived him he caught the reflection of a sudden white gleam on the dark surface of the water. At the same time something told him to dive, and dive he did. With the rocket-like speed that was his, he shot straight into the water, then away beneath the surface. He rose some ten yards downstream. After one deep, silent breath, he grasped a red mangrove branch for support, then paused to listen. He did not listen long, for there came a sudden wild swirl of water close beside him.